The metabolic activity of the small intestine in vivo is being studied in an auto-perfused preparation of rat jejunum in situ recently developed in this laboratory. The rates of uptake and the metabolic fate of nutrients taken up by intestine from the blood or absorbed from the lumen are being quantitatively determined by enzymatic, chromatographic, and radiochemical analyses. Of recent interest are the following: (a) the remarkably high rate of plasma glutamine metabolism; (b) the low rate of plasm glucose metabolism in vivo as compared with rates in vitro; (c) the extensive metabolism by intestine of several dietary amino acids absorbed from the lumen, including glutamine, glutamate, aspartate, and arginine. In related studies, the enzymes of glutamine metabolism in intestine are under investigation. A highly-active phosphate-dependent glutaminase has been localized in mitochondria of both villous and crypt cells from all regions of the small intestine. The development of the enzyme is being investigated and its properties determined, including its response to inhibitors that may be useful in determining the role of glutamine in this tissue.